Pipeline function err

Question

if pipeline function doesnot return anything i want to return 'UNK'.
but when i am trying to do this it return
PLS-00633: RETURN statement in a pipelined function cannot
contain an expression
CREATE OR REPLACE FUNCTION ALL_DEV.ROW_GENERATOR(EVENT_END_TM VARCHAR2,EVENT_START_TM VARCHAR2,OTH_MSISDN VARCHAR2,SER_MSISDN VARCHAR2) RETURN SUBS_TYPE_KEY PIPELINED AS
BEGIN
--DBMS_OUTPUT.PUT_LINE('EVENT_START_TM::'||to_char(EVENT_START_TM,'dd-mm-yyyy hh:mi:ssam'));
FOR r_emp IN (SELECT distinct A.col_3,A.col_1,A.col_4,A.col_2,B.SUBS_TYPE_KEY SUBS_TYPE_KEY
FROM temp_cdr_grouping_col A,all_dev.DIM_SUBS_IN B
WHERE A.col_4=B.SERVED_MSISDN
AND A.col_4=SER_MSISDN
AND A.col_3=OTH_MSISDN
AND col_2=EVENT_START_TM
AND col_1=EVENT_END_TM
) LOOP
PIPE ROW ( subs_key(r_emp.Subs_TYPE_KEY) );
END LOOP;
RETURN;
exception
when others then
RETURN 'UNK';
END row_generator;

Answer 1

A cursor loop like that won't cause a NO_DATA_FOUND exception, so you'll just have to count your record and return your value if the count is 0.
CREATE OR REPLACE FUNCTION ALL_DEV.ROW_GENERATOR(EVENT_END_TM VARCHAR2
                                                ,EVENT_START_TM VARCHAR2
                                                ,OTH_MSISDN VARCHAR2
                                                ,SER_MSISDN VARCHAR2) RETURN SUBS_TYPE_KEY PIPELINED AS
  v_cnt NUMBER := 0;
BEGIN
  --DBMS_OUTPUT.PUT_LINE('EVENT_START_TM::'||to_char(EVENT_START_TM,'dd-mm-yyyy hh:mi:ssam'));
  FOR r_emp IN (SELECT distinct A.col_3,A.col_1,A.col_4,A.col_2,B.SUBS_TYPE_KEY SUBS_TYPE_KEY
                FROM temp_cdr_grouping_col A,all_dev.DIM_SUBS_IN B
                WHERE A.col_4=B.SERVED_MSISDN
                AND A.col_4=SER_MSISDN
                AND A.col_3=OTH_MSISDN
                AND col_2=EVENT_START_TM
                AND col_1=EVENT_END_TM
                ) LOOP
    v_cnt := v_cnt + 1;
    PIPE ROW ( subs_key(r_emp.Subs_TYPE_KEY) );
  END LOOP;
  IF v_cnt = 0 THEN
    PIPE ROW ('UNK');
  END IF;
  RETURN;
END row_generator;This assumes that r_emp.subs_type_key is of varchar2 type and allows from 3 or more characters.